Hydrogen has attracted attention as an alternative energy source to petroleum in recent years, and the automobile equipped with a fuel cell is under development. The fuel cell vehicle is essentially equipped with a gas detection apparatus capable of detecting hydrogen gas both rapidly and accurately to detect leakage of hydrogen from the fuel cell or a hydrogen tank.
As a gas detection apparatus for detecting a flammable gas such as hydrogen gas, a catalytic combustion-type gas detection apparatus is conventionally provided with a sensing element, a voltage application unit, and a gas detection circuit. (See, for example, Japanese Unexamined Patent Application Publication No. 1998-90210) The sensing element comprises a heating unit formed of a coiled platinum wire, and a combustion unit formed into a film on the surface of the heating unit where a catalyst is dispersed. The voltage application circuit is configured to apply a heater voltage to the heating unit for heating the combustion unit by heat generated by the applied heater voltage at the heating unit. The gas detection circuit is configured to detect the flammable gas based on a change in a resistance of the heating unit (formed of the platinum wire) caused by combustion heat when a flammable gas ignites on the surface of the combustion unit.
In the catalytic combustion-type gas detection apparatus mentioned above, platinum is generally utilized as materials of the heating unit and a resistance-temperature detector. The platinum has a resistance of several ohms (Ω) to several tens of ohms (Ω). The heater voltage is set to be applied to the heater unit at about 0.2 V to 2 V so as to attain a sufficient detection voltage while saving electrical power consumption.
Since the voltage of the battery installed in the vehicle is 12 V or 24 V, the circuit voltage of the gas detection apparatus is set at 5 V or 12 V in use of the catalytic combustion-type gas detection apparatus installed on a fuel cell vehicle. For generation of the heater voltage (about 0.2 V to 2 V) to be supplied to the heating unit by the voltage application circuit, the circuit voltage of 5 V or 12 V needs to be stepped down by a series regulator or a switching regulator circuit. The step-down of the voltage by the series regulator causes a problem of wasteful use of electrical power by control elements and resistors comprising the series regulator circuit. In addition, the switching regulator circuit necessitates a complex circuit configuration having a large number of components as well as a large space for mounting thereon an inductor, being responsible for mounting cost of the apparatus.
The gas detection apparatus installed on the fuel cell vehicle is configured to stop the engine, depending on its detection result. In order to ensure sufficient safety while preventing engine shutdowns due to detection errors, the gas detection apparatus needs to detect hydrogen gas with high accuracy in a short time, for reducing electrical power consumption without badly affecting its ability to detect the hydrogen gas.